This disclosure relates generally to a method and system for reducing nitrogen oxides (NOX) and for regenerating particulate filters.
Generally, diesel engines release more undesirable NOX per kilometer into the atmosphere than gasoline engines. This is because diesel engines generally operate at higher flame temperatures in order that the diesel fuel might burn. Reduction of the flame temperatures can lead to significant increases in hydrocarbons, carbon monoxide, soot, and engine power. For at least these reasons, the flame temperatures are generally not reduced, thereby producing the NOX. The reduction of NOX into nitrogen (N2) by an exhaust after treatment system is increasingly difficult as the air fuel ratio increases. The reduction of NOX, e.g., nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O), in exhaust fluid is a widely addressed problem as a result of environmental concerns and mandated government emissions regulations, particularly in the transportation industry.
One proposed solution for the reduction of NOX is the use of a three-way conversion catalyst, which can be employed to treat the exhaust fluids. Such three-way conversion catalysts, containing precious metals such as platinum, palladium, and rhodium, can effectively use unburned hydrocarbons and carbon monoxide as reducing agents for the chemical reduction of NOX in exhaust fluids, provided that the engine is operated around a balanced stoichiometry for combustion (also referred to as “combustion stoichiometry”). The stoichiometric point depends on the fuel. For example, the balanced combustion stoichiometry for gasoline and diesel is generally at an air to fuel ratio between about 14.4 to about 14.7. However, fuel economy and global carbon dioxide emission concerns have made engine operation under lean-burn conditions desirable in order to realize benefits in fuel economy. Under such lean-burn conditions, the air-to-fuel ratio may be greater than the balanced combustion stoichiometry, i.e., greater than about 14.7, and may be between about 19 to about 35. When lean-burn conditions are employed, three-way conversion catalysts are generally efficient in completely oxidizing the unburned hydrocarbons and carbon monoxides into carbon dioxide and water. However, three-way conversion catalysts are generally inefficient in the reduction of NOX.
Another approach for treating NOX in exhaust fluids is to incorporate a NOX adsorber, also referred to as a “lean-NOX trap,” in the exhaust lines. The NOX adsorber promotes the catalytic oxidation of NOX by utilizing catalytic metal components effective for such oxidation, such as precious metals. The formation of NO2 is generally followed by the formation of a nitrate when the NO2 is adsorbed onto the catalyst surface. The NO2 is thus “trapped”, i.e., stored, on the catalyst surface in the nitrate form. The system can be periodically operated under fuel-rich combustion to regenerate the NOX adsorber. During this period of fuel-rich combustion, the absence of oxygen and the presence of reducing agents promote the reduction and subsequent release of the stored nitrogen oxides as nitrogen and water. However, this period of fuel-rich combustion may also result in a significant fuel penalty.
A more active approach, such as urea injection upon selective reduction catalyst (SCR), can also be used for NOX control. Urea selective catalyst reduction reduces the NOX emissions in diesel engines by atomizing and dispersing aqueous urea in the flowing exhaust fluid stream. In order to effectively reduce the NOX emissions, the urea selective catalyst reduction system contains components that accurately meter the aqueous urea into the exhaust fluid stream and that homogeneously disperse it in order to achieve maximum catalyst utilization. These components, in addition to the urea supplying components, however, add bulk and size to the device and thereby restrict its usage to heavy-duty applications. Further, the added complexity of the urea injection and the lack of a urea distribution infrastructure are significant detractors to using urea injection.
In addition, the urea selective catalyst reduction system is generally operated at elevated temperatures of greater than about 200° C., since urea can undergo polymerization and polymerized urea cannot be decomposed below 200° C. In some applications, such as stop and go city driving, the exhaust temperature does not even reach 200° C. Excess hydrocarbon must therefore be post-injected into the exhaust stream to elevate the exhaust temperature in order to decompose the polymerized urea. The use of such elevated temperatures generally causes an undesirable amount of additional fuel to be consumed in the operation of the selective catalyst reduction system.
In view of the aforementioned drawbacks with the three-way conversion catalyst, the lean-NOX trap, and the urea selective catalyst reduction system, it is desirable to have an onboard method and system of a size suitable for use in light duty diesel applications e.g., passenger cars, for purposes of reducing NOX emissions as well as for regenerating particulate filters.